Method of manufacturing a deep-drawing steel strip or sheet

ABSTRACT

In the manufacture of steel strip or sheet, suitable for use as deep-drawing steel for the manufacture of can bodies by deep-drawing and ironing, a low-carbon steel is provided in the form of a slab, the slab is rolled in the austenitic region to reduce its thickness to a transfer thickness, the rolled slab is cooled having the transfer thickness into the ferritic region, and the rolled slab is rolled in the ferritic region to a finished thickness. To provide a steel having reduced tendency to &#34;earing&#34; in can body manufacture, the transfer thickness is less than 1.8 mm and the total thickness reduction in the ferritic region from the transfer thickness to the finished thickness is less than 90%.

FIELD OF THE INVENTION

The invention relates to a method for the manufacture of a steel stripor sheet which is suitable as deep-drawing steel for the manufacture offor example steel can bodies by deep-drawing and ironing. Ironing isalso sometimes called wall-thinning.

DESCRIPTION OF THE PRIOR ART

To be suitable as deep-drawing steel, a grade of steel must fulfil anumber of requirements, several important ones of which are discussed inthe following.

To obtain a closed, so-called two-piece can, the first piece of whichcomprises the body including the base and the second piece is the lid, aflat blank of deep-drawing steel is taken for the first piece which flatblank is first deep-drawn into a cup with a diameter of, for example 90mm and a height of, for example 30 mm, and which cup is then ironed intothe can with a diameter of, for example 66 mm, and a height of, forexample 115 mm. Indicative values for the thickness of the steelmaterial in the different production stages are: starting thickness ofthe blank 0.26 mm, base thickness and wall thickness of the cup 0.26 mm,base thickness of the can 0.26 mm, wall thickness of the can at halfheight 0.09 mm, thickness of the top edge of the can 0.15 mm.

As this example shows, for making cans deep-drawing steel must have goodformability and must retain this property over time too, to allow forstorage and transport. In other words, deep-drawing steel must not besusceptible to ageing. Ageing leads to high forming forces, crackingduring forming and surface defects from stretcher strains. A way ofcountering ageing is so-called over-ageing, wherein carbon, thatcontributes to a great extent to ageing symptoms, is separated in acontrolled manner and can no longer diffuse to dislocations in thesteel.

The desire to save material by being able to use increasingly lightercans also acts on the requirement for high formability in order, from agiven starting thickness to the blank, to be able to achieve thesmallest possible finished thickness of the can wall and also of the topedge of the can. The top edge of the can places particular requirementson the deep-drawing steel. After the can has been formed by ironing, thetop edge is reduced in diameter, so-called necking, in order to enableuse of a smaller lid and so save on lid material. After necking, aflange is applied along the top of the top edge to enable the lid to beattached. The necking in particular and applying the flange areprocesses which place high requirements on the additional formability ofthe deep-drawing steel that was already formed earlier when the body wasbeing made.

Besides the formability the purity of the steel is important. Purity istaken to mean the degree of absence of mainly oxidic or gaseousinclusions. Such inclusions occur in steel making in an oxygen steelplant and from the casting powder that is used in the continuous castingof the steel slab which is the base material for the deep-drawing steel.In the case of necking or forming of the flange, an inclusion can giverise to a crack which itself is the cause of a later leak in the canwhen filled with contents and closed. In the case of storage andtransport, contents leaking out of the can may cause damage, inparticular by contamination, to other cans or goods in its vicinityexceeding many times the value of the leaking can and its contents. Themore the thickness of the edge of the can is reduced, the more becomesthe risk of a crack as a consequence of an inclusion. Therefore,deep-drawing steel must be free from an inclusions. In as much asinclusions are unavoidable with the present method of steel making,these should be small in size and occur only in very small amounts.

Another requirement relates to the degree of anisotropy of thedeep-drawing steel. In the manufacture of a deep-drawn/ironed orwall-thinned two piece can, the top edge of the can does not extend in aflat plane, but rather it displays a wave pattern around thecircumference of the can. In the industry the wave peaks are known asears. The tendency to form ears ("earing") is a consequence ofanisotropy in the deep-drawing steel. The ears must be cut back to thelowest trough in order to obtain a top edge laying in a flat plane whichcan be formed into a flange, and this results in material loss.

From considerations of process operation it is usual to start with ahot-rolled sheet or strip with a thickness of 1.8 mm or more. With anapproximately 85% reduction this arrives at a final thickness ofapproximately 0.27 mm. In connection with the search for less materialuse per can, a smaller finished thickness, preferably smaller than 0.21mm, is desired. Standard values of approximately 0.17 mm have alreadybeen named. Thus for a given starting thickness of approximately 1.8 mma reduction of over 90% is required. With the conventional carbonconcentrations this leads to a heavy ear formation, the cutting away ofwhich leads to extra material loss and negates a part of the benefit ofsmaller thickness. A solution is sought in using extra or ultra lowcarbon steel (ULC steel). Such steel with normally acceptable carbonconcentrations of below 0.01% to values of 0.001% or less is made in anoxygen steel plant by blowing more oxygen into the steel bath andcombusting more carbon. After this, if desired a vacuum ladle treatmentcan follow for further reducing the carbon concentration. By supplyingmore oxygen into the steel bath undesired metallic oxides also form inthe steel bath which remain as inclusions in the cast slab and later inthe cold-rolled strip. The effect of inclusions is amplified by thesmaller finished thickness of the cold-rolled steel. As discussed,inclusions are detrimental because they can lead to cracking. As aconsequence of the smaller finished thickness, this disadvantage is themore applicable of ULC steel. The result is that the yield of ULC steelgrades for packaging purposes is low because of the high volume ofrejection.

EP-A-521808 describes a process of producing a steel intended for use inmaking cans, having in the example given a final thickness of 0.18 mm.The process involves hot-rolling in the austenitic region followed bycold-rolling, with a reheating to for example 660° C. between twocold-rolling stages. The steel used has a carbon content of 0.005 to0.15%. No thicknesses of the steel in the austenitic rolling arementioned.

EP-A-504999 describes a process in which a slab is continuously cast ata thickness, following "squeezing" before the core is solidified, of 45mm. In a single roll stand, this thickness is reduced to 15 mm.Subsequently this slab may be re-heated, and it may then be coiled. Itis thereafter rolled in a continuous rolling, first in the austeniticregion to 1.5 mm and then in the ferritic region to 0.7 mm. Such a steelappears to be too thick for use as a deep-drawing steel for can bodies.

EP-A-0 370 575 describes a process for the manufacture of formable steelstrip in which molten steel is continuously cast into a slab of lessthan 100 mm, which slab is then, if desired after prereduction, cooledinto the ferritic region and in that region rolled to a final thicknessof between 0.5 and 1.5 mm.

EP-A-0 306 076 describes a method for the manufacture of formable steelstrip in which, in a continuous process a slab having a thickness ofless than 100 mm is cast, which slab is rolled in the austenitic regionto a strip of a thickness of between 2 and 5 mm. This strip is cooleddown into the ferritic region above 300° C. and rolled in that region toa final thickness of between 0.5 and 1.5 mm.

SUMMARY OF THE INVENTION

The object of the invention is to provide a method for the manufactureof a deep-drawing steel from steel grades of low carbon steel,particularly steels having carbon contents of between 0.1% and 0.01%. Bythe method it is possible, with a high material yield, to achieve asmall finished thickness, and other advantages may also be attained.

In accordance with the invention there is provided a method for themanufacture of steel strip or sheet, suitable for use as deep-drawingsteel for the manufacture of can bodies by deep-drawing and ironing,comprising the steps of

(i) forming a liquid low carbon steel into a cast slab having athickness of less than 100 mm by means of a continuous casting machine,

(ii) rolling the slab in the austenitic region while making use of thecasting heat to reduce its thickness to a transfer thickness,

(iii) cooling the rolled slab from step (ii) having the transferthickness into the ferritic region,

(iv) rolling the rolled slab from step (iii) in the ferritic region to afinished thickness,

wherein the transfer thickness is less than 1.5 mm and the totalthickness reduction in the ferritic region from the transfer thicknessto said finished thickness is less than 90% and more than 75%. The stripor sheet made by this method has the advantage of a reduced tendency toearing in subsequent deep-drawing and ironing. The degree of anisotropydepends on the carbon concentration and the total rolling reductionwhich the deep-drawing steel underwent in the ferritic region.

The invention is based on the further insight that the total reductionin the ferritic region following transition from the austenitic range isimportant for the ear formation, and that ear forming can be preventedor limited by keeping the reduction in cold-rolling in the ferriticregion within a given limit with a given carbon content by entering theferritic range with a sufficiently thin strip.

In a preferred embodiment of the method in accordance with the inventionthe total reduction by rolling in the ferritic region is not more than88%, more preferably not more than 87%. The degree of rolling reductionwhereby the minimum of anisotropy occurs is dependent on the carbonconcentration and is the greater the smaller the carbon concentration.In the case of low carbon steel the cold-rolling reduction for minimumanisotropy and so for minimum ear formation, is in the range of lessthan 87% or more preferably less than 85%. In connection with goodforming properties it is preferable for the total reduction to be morethan 75% and more preferably more than 80%. the finished thickness ofthe steel may be less than 0.20 mm, even less than 0.15 mm.

The reduction to be carried out in the ferritic region may be kept smallwith a small finished thickness in the case of a preferred form of theinvention in which the transfer thickness is less than 1.5 mm.

By the method of the invention a deep-drawing steel is provided that maybe manufactured using generally known technology and with a generallyknown apparatus and which makes it possible to manufacture thinnerdeep-drawing steel than it was possible to achieve until now. Inparticular, known techniques may be used for the rolling and furtherprocessing in the ferritic range.

It is conventional to manufacture a steel strip by starting with a caststeel slab with a thickness of between 50 mm and 250 mm, varyingaccording to the casting technology available in practice. Such a methodmay be used in this invention. Possibly following a pre-reduction, sucha cast slab is cooled to ambient temperature, temporarily stored andpossibly repaired, then reheated to within the austenitic range. Theslab is hot-rolled in the austenitic range to a desired transferthickness. With conventional processes in practice this is 1.8 mm orgreater. The slab is then rolled in the ferritic range into the strip ofthe desired finished thickness.

In a preferred embodiment of the method in accordance with the inventionthe steel strip is manufactured by continuously casting moltenlow-carbon steel into a slab and rolling said slab in the austeniticregion to the transfer thickness, without cooling said slab out of theaustenitic region. Preferably this method makes use of the casting heatin a continuous process, i.e. the steel as a whole is not subjected toreheating at least until the transfer thickness is reached, except forany heat generated by the rolling processes.

This embodiment produces the advantage that the number of individuallyseparate process stages is small. This leads to a higher material yieldbecause the run-in and run-out stages are eliminated. Moreover, when useis made of the casting heat in the slab for the rolling in theaustenitic range, a higher energy yield is attained. Furthermore,because the method has a greater degree of continuity, it can be carriedout with a more lightly built installation. In this context a continuousprocess is also understood to include a process in which the steel slabis temporarily stored in a coiling apparatus, also known as a coil-box,in the austenitic range and thus while making use of the casting heat.

A problem when hot-rolling a slab is that during the rolling thetemperature of the slab drops due to radiation loss and heat dissipationto the cooled rolls. A temperature drop under the austenitic range isundersirable on account of the quality requirements and controllabilityof the rolling process; any increase in the entry temperature, to avoidrunning beneath the austenitic range, is restricted by an acceleratedoxide formation. Increasing the rolling speed is limited because of thetendency of the strip to start flying. To ensure that the slab can berolled fully to the stated transfer thickness in the austenitic range,in a preferred embodiment of the method the slab on solidification afterthe continuous casting has a thickness of less than 100 mm, and step(ii) above comprises rolling the slab in the austenitic region into anintermediate slab, coiling the intermediate slab in a coiling apparatus,subjecting the intermediate slab to temperature homogenisation in atleast one of a furnace apparatus arranged prior to the coiling and thecoiling apparatus, and rolling the intermediate slab, after uncoilingfrom the coiling apparatus, in the austenitic region to the transferthickness.

With the furnace apparatus such as an induction furnace, heat loss thatoccurs mainly on the surface may if appropriate be compensated. Ifrequired, heat may also be removed, if the furnace is equipped forcolling. Alternatively the furnace may provide for temperaturehomogenization. In the coiling apparatus a further temperatureequalisation takes place between the surface of the slab and the core ofthe slab. The slab is also homogenised in the direction of its width fora better profile and better homogeneity of properties.

It will be clear to the expert that even when using only a furnaceapparatus or only a coil-furnace at least a part of the this advantagecan be attained, and than the invention is not limited to thecombination of these two.

Because of the number and size of the reduction stages to be carried outin the austenitic range, it is advantageous to carry out the method sothat the intermediate slab has a thickness of between 5 and 25 mm, morepreferably 5 and 20 mm. This makes possible an optimum in the number ofmill stands in a roughing installation located before the coilingapparatus and a temper-rolling installation located after it, and in therolling capacity to be installed.

Of particular advantage is an embodiment of the method in which anon-oxidising gaseous atmosphere is maintained on the surface of thesteel for at least part of the time it is in the austenitic range. Aserious problem with rolling in the austenitic range is that oxideformation on the surface of the slab occurs more quickly the more thetemperature increases, ultimately imposing a limit on the maximum entrytemperature for the austenitic rolling. By processing the slab at leastin part in a non-oxidising gaseous atmosphere, the formation of an oxidelayer is in any event limited. This means that a higher entrytemperature or a shorter period of stay may be selected in theaustenitic range. Consequently it is possible in a relatively simplemanner to achieve the desired transfer thickness of less than 1.8 mm andeven of less than 1.3 mm. On a small scale it has been found possible toachieve transfer thicknesses of around 1.0 mm.

In a particularly effective embodiment of the method in accordance withthe invention a non-oxidising gaseous atmosphere is maintained in atleast one of the furnace apparatus and the coiling apparatus, or inboth. In a conventional furnace apparatus the slab is exposed to thesurrounding gaseous atmosphere for a relatively long time and withoutbeing protected. Making this gaseous atmosphere non-oxidizing achievesthe effect that at least in the furnace apparatus less oxide forms ornone forms at all. The coiled slab stays in the coiling apparatus for arelatively long time at a relatively high temperature. Maintaining anon-oxidizing atmosphere in the coiling apparatus achieves the effectthat no oxide scale can form which otherwise might be considerable,especially because of the high temperature of the slab.

The invention can be carried out in a plant for the manufacture of steelstrip or sheet, having

(a) a continuous casting machine for casting a steel slab,

(b) a furnace apparatus for adjusting the temperature of the slab fromthe continuous casting machine, having an enclosure having an entryport, an exit port and a path for traversing by the slab from the entryport to said exit port, said enclosure maintaining a desired atmosphereat the path,

(c) a coiling apparatus for coiling the slab from the furnace apparatus,having an enclosure providing an enclosed space for a coil andmaintaining a desired atmosphere in the enclosed space, the enclosure ofthe coiling apparatus having an entry port for the slab,

(d) an austenitic rolling apparatus for rolling the slab to a transferthickness in the austenitic region, after uncoiling from the coilingapparatus, and

(e) a ferritic rolling apparatus for rolling the slab having saidtransfer thickness in the ferritic region into strip or sheet having adesired final thickness,

wherein the exit port of the furnace apparatus is substantiallygas-tightly and detachably connectable to the entry port of said coilingapparatus. The plant may also have means for reducing the thickness ofthe slab between the continuous casting machine and the furnaceapparatus.

Preferably the plant has means for providing a non-oxidising atmospherein contact with said slab in at least one of the furnace apparatus andthe coiling apparatus.

Such as apparatus and its advantages and specific embodiments aredescribed in the International patent application "Plant for themanufacture of steel strip" with the same filing date as the presentapplication and in the name of the same applicant, with reference no. HO848. The content of that application is deemed to be included in thepresent application by this reference.

Typically the furnace apparatus is built as an electric furnace inwhich, by means of resistance or inductive heating, energy is suppliedto the slab, so that the surface of the slab is heated again afterhaving cooled as a consequence of the descaling by high pressure watersprays and because of heat loss to the surroundings. In the case ofconventional plants, during this heating the surface is exposed to thenormal outside atmosphere along a relatively great distance and thus fora relatively long time, so that an oxide scale again forms on thesurface, which under these conditions is a thin, tenacious layer whichin practice cannot be completely removed with available very high waterpressures and which ultimately must be removed by pickling.

The furnace apparatus may be employed only for homogenizing thetemperature of the steel slab, or may be arranged to alter at least thecore of the slab in temperature.

In the plant so provided, the slab is prevented from coming into contactwith the outside atmosphere as it passes through even a relatively longfurnace apparatus, so that oxide scale thereby forming on the outersurface of the slab is minimized.

As stated, the coiling apparatus is provided an enclosure, i.e.screening means, for maintaining the desired gaseous atmosphere in thecoiling apparatus. In the case of a conventional plant, the slab iscoiled at a relatively high temperature in the coiling apparatus andstored there for some time for temperature homogenising or for waitingfor further processing in the rolling apparatus. With the plant, whenthe coiling apparatus has a non-oxidizing atmosphere, the slab isprevented from oxidising or oxidising further during its stay in thecoiling apparatus. The coiling apparatus preferably has sealing means,such as a door for closing its entry port and maintaining the desiredatmosphere in it, when it is detached from the furnace apparatus.

As mentioned, in the plant the exit of the furnace apparatus is coupledessentially gas-tightly and attachably to the coiling apparatus. Thisalso achieves the benefit that from the time when it runs into thefurnace apparatus until it is conveyed out of the coiling apparatus theslab does not come into contact with the outside air, but rather it iscontinually surrounded by a gaseous atmosphere of a desired composition.For this the gaseous atmosphere in the furnace apparatus and in thecoiling apparatus may be the same or different.

Preferably the coiling apparatus is mobile and is movable from aposition of connection to the furnace apparatus to a position foruncoiling of said slab into the austenitic rolling apparatus. This alsominimizes time of contact with the ambient atmosphere.

The slab uncoiled from the coiling apparatus is rolled in a followingfinishing train into a hot-rolled strip with a thickness smaller than1.8 mm, preferably smaller than 1.5 mm.

In order to keep the finishing train as simple as possible and as smallas possible, and to limit the exit speed from the finishing train, it ispreferable to make the thickness of the uncoiled slab as small aspossible. In order to be able to coil this slab well, it is preferablefor the coiling apparatus to be provided with a mandrel onto which thecoil can be coiled. The crop end of a slab, whether or not roughened, isclamped onto the mandrel and then coiled in the coiling apparatus intothe coil in a path determined by the mandrel. This forced path makes itpossible to coil a wide range of thicknesses reliably. This achieves agreat freedom in that part of the process taking place prior to coilingand it is also possible to coil thin, rolled slabs.

A conventional plant for the further processing of a hot-rolled stripcomprises separate apparatuses for cold-reducing and annealing. For thinand mechanically strong cold-rolled steel, a once cold-rolled strip isannealed a first time and then again cold-rolled, annealed andtemper-rolled, so-called double-cold-reduced steel (DCR).

The plant makes it possible to manufacture a hot-rolled strip of lessthan 1.3 mm in thickness. Such a strip may be further processedeffectively in a cold-rolling apparatus which is provided, insuccession, with a first cold-rolling train, a recrystallisation furnaceand a second cold-rolling train. Because the starting material is thinhot-rolled strip, the apparatus may be built as installations placed insuccession through which the strip to be processed runs in anessentially continuous process. This results in compact installationwhich moreover makes it possible to make DCR steel in a continuousprocess. Such DCR steel and its applications, are known, such as forexample three-piece cans in the packaging industry.

For obtaining good forming properties it is preferable for the firstcold-rolling train to be suitable for a reduction of at least 30% in onepass in at least one of the mill stands of the first cold-rolling train.With such a reduction sufficient deformation is applied in the steel forsubsequent recrystallisation. In addition it is possible to reduce thematerial far enough that, following recrystallisation, it is possible toroll to the finished thickness with relatively simple mill stands.

A particularly compact and easily controlled apparatus is obtained withan embodiment in which the first cold-rolling train comprises three4-high mill stands.

Good forming properties with a desired reduction are also to be achievedwith an embodiment of the apparatus in which the second cold-rollingtrain comprises two mill stands, preferably two 6-high mill stands,although two 4-high stands are possible too.

The second cold-rolling train is preferably suitable for a reduction toa finished thickness smaller than 0.14 mm. This produces the advantagethat it can be used to manufacture a cold-rolled strip or sheet in avirtually continuous process with a thickness which can otherwise onlybe achieved with the complicated technique of double-cold-rolling.

It will be clear to the expert that the compact installation comprisingthe first cold-rolling train, the recrystallisation furnace and thesecond cold-rolling train can also be used as an autonomous apparatus,or in a combination with an apparatus for the manufacture of anaustenitically hot-rolled strip other than that described in thisapplication. The compact installation is capable of making DCR grades ofsmall thickness for known applications such as packaging material with athickness of 0.14 mm or less.

INTRODUCTION OF THE DRAWINGS

The invention will be illustrated in the following by means of adescription of a non-limitative example of a plant for carrying out theinvention, with reference to the drawings.

In the drawings:

FIG. 1 is a schematic top-view of part of a plant for carrying out theinvention,

FIG. 2 is a schematic side-view of the plant of FIG. 1, and

FIG. 3 is a schematic side view of a further part of the plant forcarrying out the invention.

DESCRIPTION OF THE EMBODIMENT

FIG. 1 shows a continuous casting machine 1 for two stands. Thecontinuous casting machine 1 comprises a ladle turret 2 in which twoladles 3 and 4 can be accommodated. Each of the two ladles can containapproximately 300 tons of liquid steel. The continuous casting machineis provided with a tundish 5 which is filled from the ladles 3 and 4 andkept filled. The liquid steel runs out of the tundish into two moulds(not drawn) from where the steel, now in the form of a partiallysolidified slab with its core still liquid, passes between the rolls ofcurved roller tables 6 and 7. For some grades of steel it can be anadvantage to reduce the steel slab in thickness in roller tables 6 and 7while its core is still liquid. This is known as squeezing.

Descaling sprays 8 are located on the exit side of the two roller tables6 and 7, by which oxide scale is sprayed from the slab with a waterpressure of approximately 200 bar. Starting with a cast thickness of forexample approximately 60 mm, the slab typically still has a thicknessfollowing squeezing of approximately 45 mm. By the 3-stand roll trains 9and 10 the slab is further reduced to a thickness ranging from 10 to 15mm. If desired the head and the tail may be cut off the slab by theshears 11 and 12, or the slab sheared into parts of a desired length.

Instead of casting a thin slab with a thickness of less than 100 mm, itis also possible to cast a thicker slab and by means of rolling, inparticular by means of reversible rolling, to reduce the thickness ofthe slab to a value ranging from 10 to 15 mm.

In the method of the present invention the slab will generally be rolledinto an intermediate slab with a thickness of 10 to 15 mm, as mentionedabove. This rolled slab is conveyed into the furnace apparatus 13 or 14.The furnace apparatuses are each provided with heating means (notdrawn), for example induction heating means, for heating the rolled slabup to a desired temperature in the austenitic region. The furnaceapparatuses are in the form of enclosures and are provided withconditioning means for creating and preserving a desired non-oxidizinggaseous atmosphere in the furnace apparatus. In the embodiment shown theconditioning means of a furnace apparatus comprise a suction line 15, apump 17, gas metering and gas scrubbing means 19 and a supply line 21along which the gas is pumped into the furnace apparatus. If desired thegas metering and gas scrubbing means 19 may also comprise a gas heatingapparatus for compensating for any heat loss. Thus heat exchangers canbe employed to control the gas temperature, using gas combustion tosupply heat, and water for cooling.

The gas atmosphere provided in the furnace apparatus and preferably alsoin the coiling apparatus is substantially non-oxidizing, thoughinevitably it may include a small amount of oxygen due to leakage ofair. Preferably it is based on nitrogen, although an inert gas such asargon may be used it its high cost allows. The nitrogen may containadditive for inhibiting nitriding of the steel surface, as is known inthe process of batch annealing of steel. The gas atmosphere may containwater vapour.

The furnace apparatus is provided on its entry and exit sides with ports23, 25 having sealing means to substantially prevent any undesiredpenetration of gas from the surrounding atmosphere. A suitable value forthe temperature of the reduced slab on exiting the furnace apparatus is1080° C. The furnace apparatus is coupled essentially gas-tightly to thecoiling apparatus 27, which coiling apparatus 27 itself comprises anessentially gas-tight enclosure in which the slab is coiled into a coil.The coiling apparatus is preferably provided with a mandrel 29 whichsupports the coil as it is being coiled.

In this embodiment, the gas atmosphere provided in the furnace apparatusalso enters the coiling apparatus when the latter is connected to it.Alternatively both the furnace apparatus and the coiling apparatus maybe provided with conditioning means, as described above, for providingthe desired atmosphere.

As appropriate, virtually synchronously with coiling of a slab ontocoiling apparatus 27, a slab cast on the other strand is coiled incoiling apparatus 28 provided with a mandrel (not drawn). Coilingapparatuses 27 and 28 and furnace apparatuses 13 and 14 are eachprovided with sealing means 33, 35, 34, 36 respectively, by which thecoiling apparatuses and the furnace apparatuses may be sealed foruncoupling, so that following uncoupling no gas can penetrate from theoutside atmosphere and the gaseous atmosphere in the coiling apparatusesand the furnace apparatuses remains preserved.

The sealing means for the ports of the furnace apparatuses and thecoiling apparatuses are suitably steel flaps, biassed to the closedposition, or they may be doors which are driven. To minimize gasleakage, flexible curtains may additionally be provided.

As soon as the coiling apparatus 27 is filled with a slab coiled into acoil, this coiling apparatus 27 is uncoupled from the furnace apparatus13 and driven from position A (see FIG. 1) past position B to positionC. At position C there is a turnstile 31 (not drawn) by which atposition C the coiling apparatus may be rotated through 180° around avertical axis. Following rotation the coiling apparatus is driven pastwaiting position D to entry position E. As a coiling apparatus travelsfrom position A to position E, an empty coiling apparatus is driven fromposition E to a turnstile 37 at position F. Following rotation through180° around a vertical axis by the turnstile 37, the coiling apparatusis driven past position G to the starting position A and there it isready for taking up a fresh slab.

A corresponding working method is applicable for the second strand,whereby the coiling apparatus 28 filled with a coil is driven fromposition B to position C and following 180° rotation to position D. Thecoiling apparatus stays parked in this position until a coilingapparatus which is currently uncoiling, for example coiling apparatus27, is empty at position E and driven off to the now vacated position F.As soon as coiling apparatus 28 leaves position B, an empty coilingapparatus from position I, following rotation through 180° around avertical axis by means of a turnstile 38, is moved via position K totake up the position of the coiling apparatus 28 now driven off. The newslab fed out of the furnace apparatus 14 can be coiled in the emptycoiling apparatus. Devices, preferably electrical current conductors(not shown), are fitted along the paths over which the coilingapparatuses travel for providing power for internally heating thecoiling apparatuses according to need. For this purpose, the coilingapparatus contains electrical heaters for heating the coils and contactsfor pick-up of power from the fixed conductors. Path B, C, D, E iscommon and used as described by coiling apparatuses of both strands.Position C has a rotation facility and position D is a waiting positionin which a coiling apparatus filled with a coil is ready to be moved toposition E as soon as it becomes free. Positions C and D may be swappedor may coincide.

In the manner described, a coiling apparatus 27 arrives at position Ewith its sealing means 33 closed and filled with a coil with atemperature of approximately 1080° C. After the sealing means 33 havebeen opened the extremity of the outer winding corresponding to the tailof the coiled slab is fed into the rolling train. If desired the headmay be cut off by crop shears if it does not have a suitable shape orcomposition for further processing. Should some oxide still haveoccurred, this can then be removed easily using the high pressure spray42. In practice oxide formation will be negligible because the slab hasbeen almost constantly in a conditioned gaseous atmosphere. Because thecoiling apparatus rotates through 180°, its original infeed which is nowthe outfeed can be brought up very close to the entry of the rollingtrain. This also minimizes oxide formation.

In the example shown, the rolling train 40 is provided with four millstands and is so designed that the slab can be rolled in the austeniticregion, or at least at such a temperature that only a small partconverts to ferrite. A minimum target temperature of approximately 820°C. applies for low-carbon steel. For controlling thickness, width andtemperature, a measuring and control apparatus 43 may be incorporated inthe rolling train, after or between the mill stands.

As described above, the apparatus achieves the effect that less oxideforms as the slab and the strip are being processed. Because of this andbecause of the low entry speed in the last rolling train 40 which isachieves as an additional advantage, it is possible to attain a smallerthan conventional finished thickness of the hot rolled steel. Exitthicknesses of 1.0 mm and less from the rolling train 40 can be attainedwith the plant described.

After exiting the rolling train 40, the hot-rolled strip passes througha colling line 44 in which the strip is cooled to a desired temperaturein the ferritic range by means of water cooling. Finally the strip iscoiled into a coil on the coiling apparatus 45. By selecting the coolingon the cooling line it is possible in a manner known in itself toinfluence the recrystallisation in the ferritic range and therebyinfluence the mechanical properties of the hot rolled strip.

Therefore, using the plant of FIG. 1 in this manner it is possible usingthe casting heat to manufacture in a successive series of process stagesa austenitically rolled steel strip suitable for further processingdescribed below. External heating after casting may be avoided (exceptany heat generated by the rolling).

From the coiling apparatus 45 or directly from the cooling line 44 orusing another manner of temporary storage, the hot-rolled strip isfurther processed in a cold-rolling apparatus as illustrated in FIG. 3.

FIG. 3 shows a pickling line 50 through which the strip 49 is led bymeans of deflector rolls 51, 52, 53, 54 for removing any oxide thatmight have occurred. After it has exited the pickling line the stripundergoes a first series of reduction stages in the first cold-rollingtrain 55 comprising three 4-high roll stands 56, 57, 58. In one of theseroll stands, the thickness reduction is at least 30%. The strip is thenrecrystallized in the continuously operating recrystallisation furnace60 at a desired temperature. To keep the installation compact therecrystallisation furnace is built as a vertical furnace. The strip isfed into and out of the furnace by making use of deflector rolls 61, 62,63, 64. Having exited the furnace the strip may now be cooled in thecooling apparatus 65. Having been deflected around deflector roll 66 thestrip is taken for a further thickness reduction into the secondcold-rolling train 67 comprising two 6-high mill stands 68 and 69.Afterwards the strip 49 is coiled in coiling apparatus 70 or cut intopieces of desired lengths by a shearing apparatus, not drawn, of knowntype. If desired the strip may be provided with a coating prior tocoiling or shearing.

Typical values for the thickness of the strip are: on entering the firstrolling train approximately 1.0 mm, on exiting the first rolling trainapproximately 0.2 mm, on exiting the second rolling train approximately0.12 mm. This gives a reduction in the ferritic region of 88%. As statedabove, reductions of not more than 87% or even not more than 85% arepreferred, in order to reduce "earing", and are clearly feasible withthis apparatus.

What is claimed is:
 1. A method for the manufacture of steel strip orsheet, suitable for use as deep-drawing steel for the manufacture ofcans by deep-drawing and ironing, comprising the steps of:(i) forming aliquid low carbon steel into a cast slab having a thickness of less than100 mm by means of a continuous casting machine, (ii) rolling said slabin the austenitic region while making use of the casting heat to reduceits thickness of a transfer thickness, (iii) cooling the rolled slabfrom step (ii) having said transfer thickness into the ferritic region,(iv) rolling the rolled slab from step (iii) in the ferritic region to afinished thickness, wherein said transfer thickness is less than 1.5 mmand the total thickness reduction in the ferritic region from saidtransfer thickness to said finished thickness is less than 90% and morethan 75%.
 2. A method according to claim 1, wherein said total thicknessreduction in the ferritic region is less than 87%.
 3. A method accordingto claim 2, wherein said rolling in said step (iv) is at least partlycold-rolling.
 4. A method according to claim 2, wherein said step (i)comprises continuously casting molten low-carbon steel into a slab androlling said slab in the austenitic region to said transfer thickness,without cooling said slab out of the austenitic region.
 5. A methodaccording to claim 2, wherein for at least part of the time where saidslab is in the austenitic region, it is maintained in a non-oxidizinggaseous atmosphere.
 6. A method according to claim 1, wherein saidrolling in said step (iv) is at least partly cold-rolling.
 7. A methodaccording to claim 6, wherein for at least part of the time where saidslab is in the austenitic region, it is maintained in a non-oxidizinggaseous atmosphere.
 8. A method according to claim 6, wherein in saidstep (iv) the steel being rolled is passed through successively a firstcold-rolling train, a recrystallization furnace and a secondcold-rolling train.
 9. A method according to claim 4, wherein said slabon solidification after said continuous casting has a thickness of lessthan 100 mm, and said step (i) comprises rolling said slab in theaustenitic region into an intermediate slab, coiling said intermediateslab in a coiling apparatus subjecting said intermediate slab totemperature homogenization in at least one of a furnace arranged priorto said coiling and said coiling apparatus, and rolling saidintermediate slab, after uncoiling from said coiling apparatus, in theaustenitic region to said transfer thickness.
 10. A method according toclaim 8, wherein said first cold-rolling train comprises at least onemill-stand which effects a thickness reduction of at least 30% in onepass.
 11. A method according to claim 10, wherein said secondcold-rolling train effects reduction to said finished thickness which isless than 0.14 mm.
 12. A method according to claim 8, wherein saidsecond cold-rolling train effects reduction to said finished thicknesswhich is less than 0.14 mm.
 13. A method according to claim 1, whereinsaid step (i) comprises continuously casting molten low-carbon steelinto a slab and rolling said slab in the austenitic region to saidtransfer thickness, without cooling said slab out of the austeniticregion.
 14. A method according to claim 13, wherein said slab onsolidification after said continuous casting has a thickness of lessthan 100 mm, and said step (i) comprises rolling said slab in theaustenitic region into an intermediate slab, coiling said intermediateslab in a coiling apparatus subjecting said intermediate slab totemperature homogenization in at least one of a furnace arranged priorto said coiling and said coiling apparatus, and rolling saidintermediate slab, after uncoiling from said coiling apparatus, in theaustenitic region to said transfer thickness.
 15. A method according toclaim 14, wherein a non-oxidizing gaseous atmosphere is maintained in atleast one of said furnace and said coiling apparatus, while saidintermediate slab is present.
 16. A method according to claim 14,wherein said intermediate slab has a thickness in the range 5 to 20 mm.17. A method according to claim 14, wherein said intermediate slab has athickness in the range 5 to 25 mm.
 18. A method according to claim 17,wherein a non-oxidizing gaseous atmosphere is maintained in at least oneof said furnace and said coiling apparatus, while said intermediate slabis present.
 19. A method according to claim 1, wherein for at least partof the time where said slab is in the austenitic region, it ismaintained in a non-oxidizing gaseous atmosphere.